The present invention generally relates to reference standards and ultrasonic inspection methods and, more particularly, to an ultrasonic inspection reference standard for composite materials and to an ultrasonic inspection process using a polymer resin reference standard.
The use of fiber-reinforced composite materials over metals and their alloys has increased significantly over the past years because of the weight savings and the improvement in fatigue life and corrosion control. Fiber-reinforced composite materials contain a strong and stiff fiber, such as a carbon fiber, embedded in a softer matrix material, such as a resin. The resin is used as a binding agent to hold the fibers together while the fibers provide the strength. The fibers can take on several forms including, for example, tape, fabric, and may be woven in either two dimensions or three dimensions. Laminated composite materials generally exhibit an initial stiffness that is used in the design of structures. Laminated composites are constructed of many layers of fiber-reinforced materials. Fiber-reinforced polymer matrix composites, such as graphite/epoxy and carbon/cyanate ester are now the materials of choice for spacecraft and launch vehicle structures and subsystems such as optical benches, instruments, and antennas. Furthermore, fiber-reinforced polymer matrix composites are widely used, for example, in commercial and military aircraft, sports equipment, and industrial and medical equipment.
The laminated composite materials undergo non-destructive testing procedures, such as ultrasonic and radiographic inspection, during aircraft manufacturing, maintenance, and repair. Common aircraft applications include thickness testing, delamination detection, and porosity evaluation of aircraft structures. Ultrasonic testing introduces high frequency sound waves into the test material to detect subsurface discontinuities. Transducers are used both to transmit and receive sound energy into and from test material. In the process, high frequency sound in the order of 500 KHz (kilohertz) to 10.0 MHz is sent, for example, into a composite laminate and echoes from the laminate are then measured in the time domain and the amplitude domain to determine the materials quality. Through-transmission and pulse-echo techniques are most commonly used in the aircraft airframe industry, both commercial and defense. The through-transmission technique uses a sending transducer that introduces the ultrasonic energy into the test part and a receiving transducer that measures the amount of energy that exits the part at the opposite side. The amount of energy absorbed is then evaluated. The pulse-echo technique uses only one transducer that introduces short bursts of ultrasonic energy into the test part at regular intervals and that also measures the amount of energy that is reflected from an internal flaw or structural discontinuities as well as the time delay between transmission of the initial pulse and the arrival of the echo. The amount of energy reflected is a function of the size of the flaw in relation to the size of the incident beam. Some advantages of the ultrasonic testing method include high penetrating capability, high sensitivity and resolution, portability, single surface accessibility, and the immediate interpretation of test results. Reference standards are required to calibrate the test equipment to ensure the successful operation of the test equipment.
Composite reference standards are employed when performing ultrasonic inspection to establish the bulk attenuation properties for a pristine material. Typical composite standards are manufactured using fiber-reinforced resins identical to the materials used in the final product and, therefore, in the parts to be tested. The cost of building a composite reference standard include, for example, the cost to design, material cost, labor cost to lay-up the material, autoclave usage cost, post-cure machining cost and the standard qualification cost. For example, the preparation of photomicrographs, image analysis, and acid digestion, a process of dissolving material in an acid matrix that may be used for laboratory analysis, may be reflected in the standard qualification cost. Furthermore, some resin systems also require an additional cure process whereby the material returns to the oven or autoclave after its initial cure to advance the cure state of the resin. Therefore, composite reference standards are undesirable due to the cost to design, manufacture, and qualify. A typical step-wedge standard that is commonly used, for example, during the ultrasonic inspection of airframes, costs between about $2,000 and about $4,000.
As can be seen, there is a need for an alternative approach to building composite reference standards to reduce the cost of design, manufacturing, and qualification. Furthermore, there is a need for a method that enables the mass production of reference standards that can be used for ultrasonic inspection of composite materials due to the growing industrial application of composite materials and, therefore, an increased volume of composite parts to be inspected using ultrasonic inspection techniques.
There has, therefore, arisen a need to provide a reference standard that can be used in ultrasonic inspection processes for composite materials, that is inexpensive, that may be produced in high quantities, and that can replace existing expensive composite standards. There has further arisen a need to manufacture a standard for ultrasonic inspection of composite materials that has acoustic properties that are comparable to currently existing composite standards but can be manufactured at a lower cost and with a reduced machining time.